Increased expression of matrix metalloproteinases (MMPs) is associated with the progression of many tumors including malignant melanoma. The activation of MMP cascades is controlled by events at the cell surface, starting with surface expression of membrane type matrix metalloproteinases (MT-MMP) and also requiring the interactions between MMPs and adhesion receptors expressed in invasive tumor cells. The progression of primary melanomas is associated with increased expression of MT1-MMP and other soluble MMPs, including MMP-2, MMP-1 and MMP-13. MT1-MMP can activate proMMP-2 gelatinase, leading to more rapid degradation of ECM components and to activation of proMMP-1 collagenase. In the current proposal, we present evidence that numerous invasive primary melanoma cells express MT3-MMP (a transmembrane MMP related to MT1-MMP). Surface expression or MT3-MMP stimulates invasion of primary melanoma cells through native type I collagen gels in vitro and leads to increased in vitro gelatinolytic activity and accelerated tumor growth following subcutaneous injections into immunocompromised mice. MT3-MMP mediated invasion requires both MMP-2 and MMP-1, suggesting that MT3-MMP initiates an MMP activation cascade at tumor cell surfaces. Finally, MT3-MMP mediated human melanoma invasion requires the expression of Melanoma Cell Surface Proteoglycan (MCSP), a large transmembrane adhesion receptor associated with the vast majority of human melanomas. Inhibition of the MCSP core protein expression, or inhibiting the addition of chondroitin sulfate (CS) to newly synthesized MCSP core protein, inhibits melanoma invasion and gelatinolytic activity in these cells. MT3-MMP co-precipitates with MCSP in melanoma extracts, and this co-precipitation is dependent on the presence of CS on the MCSP core protein. Furthermore, recombinant MT3-MMP, MMP-2 and MMP-1 all bind to CS-coupled beads. These results suggest that MCSP may help to localize and/or activate MMP cascades on the surface of invasive melanoma cells. The current proposal has two major goals. First we will further define the relationship between surface expression of MT3-MMP and activation of proMMP-2 or proMMP-1 in melanoma invasion. Secondly, we propose to test the hypothesis that MCSP serves to bind and/or modify the activation of these three invasion-promoting proteases. Understanding chondroitin sulfate interactions with MMPs may lead to new therapies to inhibit melanoma invasion and metastasis.